1. Field of the Invention
The invention relates to a crystal oscillating device having a temperature compensation function and a method of adjusting such crystal oscillating device.
2. Description of the Related Art
A crystal oscillating device includes a crystal oscillator, and the oscillation frequency of the crystal oscillator greatly varies with varying temperatures.
In order to decrease the oscillation frequency variations of the crystal oscillator due to the aforementioned temperature variations, conventionally used is a technique for controlling the voltage to be applied to varactor diodes with a control circuit, the varactor diodes being used as frequency adjusting elements of the crystal oscillator.
An exemplary construction of such a conventional control circuit is characterized as follows. If such a circuit is designed to make temperature compensations for a temperature range of 130.degree. C. from -35.degree. to 95.degree. C., temperature compensation data are stored in a memory in 4.degree. C. increments by dividing such 130.degree. C. temperature range into 4.degree. C. segments.
In this case, the temperature compensation data in the aforementioned conventional example require data such as precise gradient, temperature bias point, polarity, rough gradient, and fixed offset in 4.degree. C. increments. These data constitute a single control voltage setting group, and a total of 32 groups is stored in the memory.
That is, the conventional technique is characterized as selecting and outputting the data belonging to a single control voltage setting group within the memory based on the temperature detected by a temperature sensor. As a result, the oscillation frequency of the crystal oscillator can be stabilized independently of fluctuations in surrounding temperature.
What imposes problems in the aforementioned conventional example is that not only an increase in the size of the memory leads to an increase in the size of a semiconductor integrated circuit that is constructed of the memory and the control circuit, but also the control circuit itself becomes complicated and therefore disadvantageously consumes more power.
That is, in the aforementioned conventional example, the temperature compensation data are stored in the control voltage setting groups within the memory every 4.degree. C. in order to make temperature compensations covering the temperature range from -35.degree. to 95.degree. C. in 4.degree. C. increments. Therefore, the memory must have such a large capacity as to accommodate a total of 32 control voltage setting groups. In addition, in order to control such a large-capacity memory having a total of 32 control voltage setting groups, the control circuit therefor tends to be complicated and large-sized. As a result, the semiconductor integrated circuit that is constructed of the memory and the control circuit becomes disadvantageously large-sized.
Further, the control circuit that controls the memory having 32 control voltage setting groups has exhibited a tendency to consuming more power.